asterisk users - Sep 2005 - Asterisk clustering with SIP proxy?

hi

i've heard it should be possible, but i can't find out how...

I want to configure a bunch of asterisk boxes to do SIP/PSTN  
connectivity, and I need SER or something to do some balancing in  
front of them. The requirements are listed below.

  * SER MUST accept and load balance incoming calls over n asterisk  
boxes (anything between 2 and 20 servers depending on installation)
  * If SER forwards a call to a server being busy or down, SER SHOULD  
retry on another server
  * SER SHOULD balance the number of calls to each server based on  
the codec used so single servers will not be overloaded by  
transcoding costs.
  * If possible, SER SHOULD be able to fail over to another SER box  
if SER fails.

Does anyone know if this is possible? I'd gladly pay someone to help  
me out here...

roy